Mar-2014

Gas chromatography trends and troubleshooting in petrochemical analysis (part 1)

Gas chromatography (GC) is at the fore in the petrochemical sector as an analytical method that can be applied across a broad range of applications.

Stephen Harrison
Linde AG

Article Summary

Analysis of chemical components for plant process control has been elevated to unprecedented levels of accuracy to ensure optimal performance. The focus is not only on the main process stream components, but also on trace impurities which have a definite influence on the process and the final product. Against this background, GC technology has advanced towards higher sensitivity, or lower detection limits, and detecting more chemical components within a sample, even if they occur in extremely small quantities.

In tandem with this trend, GC is also advancing out of the laboratory setting and into the realm of miniaturisation, with instruments small enough to be located in the refinery itself. These miniature GC systems are placed at the location where the sample is generated, making it possible to conduct an analysis without having to move the sample to the laboratory. Micro-gas chromatographs also have a positive impact on refinery running costs because they require very low flow rates of carrier gas.

In the world of complex analysis, there is also a move towards combining multiple detectors - with resultant instruments having multifaceted and highly involved configurations designed for special tasks. A specialist GC like this could analyse 30 components or more, from a single sample injection. The high sensitivity and low detection levels presently required demand ever higher purity carrier gases, often as high as a quality of 6.0 – that is, 99.9999% purity – or not having more than 1 part per million (ppm) total reported impurity level. The purity of the carrier gas is crucial for performance, maintenance and longevity of refinery GC instrumentation.

Yet another trend is the evolution of the gas chromatograph instrument itself becoming a versatile, comprehensive, “do-everything” piece of equipment. Today, a typical gas chromatograph is a ‘black box’ with everything needed to conduct a specific type of analysis built into it. In addition to the fundamental column and detector, additional examples are flow controllers, purifiers and gas pressure regulators which are all built into the gas chromatograph, so that it can be used on a ‘plug and play’ basis with maximum convenience and reliability.

Specialty gases are essential for efficient operation of a gas chromatograph. Separation takes place in the gaseous phase by introducing a sample which is transported by a carrier gas that separates the sample over the static medium in the column. Typical carrier gases for a range of applications include helium, usually supplied in cylinders, nitrogen supplied in cylinders, from liquid sources or via a gas generator, argon for niche GC applications — also supplied in cylinders — or hydrogen in cylinders or via gas generators. The choice of carrier gas depends on the type of detector, column, application and safety requirements and will also be dependent on separation efficiency or speed requirements.

Hydrogen is finding increased popularity as an effective substitute for helium, which is becoming scarce. Hydrogen also has certain benefits over helium, making its use preferable on some occasions. It has the lowest viscosity of all gases, thereby providing the highest mobile phase velocity and the shortest analysis time. Helium, on the other hand, gives the best overall performance and peak resolutions for many applications, making it an optimum choice of carrier gas where that is the primary concern.

A GC’s detector will often harness speciality gases such as a combination of air and hydrogen which are used in a flame ionisation detector (FID), or helium which is used in a helium ionisation detector (HID). Other specialty gases used in association with GC are gas mixtures to calibrate the detector in order to ensure accurate measurement, or zero gases to set a zero reading on the detector.

The information received by laboratory technicians is essentially a graph called a chromatogram, with peaks representing the different chemicals in the sample (analytes) and the volume and proximity of peaks indicating the amount of these analytes present in the sample. Each chromatogram illustrates the fingerprint of the mixture of chemicals introduced into the chromatograph from the sample, the specialty gases used and any contaminant gases unintentionally introduced from the surrounding atmosphere. With the complexity of modern GC operations, the possibility exists for things to go wrong. When errors occur they are likely to show up in the chromatogram and cause problems in the interpretation of the analytical results.

As in other industries, laboratory operators in the petrochemical sector sometimes encounter problems of the type which occasionally beset all GC users. But, in this sector, with such a broad range of analytes, so many potential impurities and such wide concentration ranges encountered during analysis, problems which invariably arise in the hydrocarbon processing sector can sometimes be difficult to resolve.

As a leading industrial gas company, Linde is committed to helping petrochemical industry GC users troubleshoot problems. This can be somewhat challenging, particularly in cases where the analysis technology is contained in one box, as in order to identify a problem, you need to examine what is going on at each stage of the process. Although the merging of a number of pieces of equipment into one GC unit simplifies the process when everything runs smoothly, it is when problems are encountered that the identification of the cause can be more complicated than in simpler systems used 20 years ago. This is due to the greater number of potential failure points contained inside the instrumentation setup.

The sulphur chemiluminescence detector (SCD) has emerged as a powerful tool in refinery GC applications. GC-SCD is used for the quantitative determination of various sulphur organic species, such as hydrogen sulphide, mercaptans, thiophenes, benzothiophenes and sulphides in hydrocarbon samples. Because sulphur speciation is essential during oil catalytic processing in a refinery, this technology is a highly sensitive and useful technique to characterise crude oils of different origin.

When it comes to troubleshooting issues associated GC-SCD technology, a key consideration for laboratory personnel is the physical properties of the sample delivery lines. It is essential that these delivery lines be constructed from an inert material, such as Hastelloy®, since the incorrect material could result in the component reacting on the walls of the line and displaying a zero result on the chromatogram. Although 316 stainless steel is the most common choice in general industry for this application, it is not appropriate for refinery analysis because certain sulphur compounds in the sample line could adhere to the walls and would therefore not reach the analyser at the same time as the bulk of the sample.